MCUs work separately but won't as multi MCU

I have 2 control boards which worked together configured as multi MCU on my previous setup with Octoprint - this was like half a year old version of klipper. Now I decided to try Mainsail and I installed everything from scratch using KIAUH - 1x Klipper, 1x Moonraker, 1x Mainsail. I loaded my printer.cfg and except updating parameters for steppers it same as before. I also updated firmware in the MCU to the latest klipper.
Problem is with this setup it does not work in multi MCU config anymore. I scratched all unecessary config out to filter out possible errors and I’m left with this, that still does not get to Ready state:

[include kiauh_macros.cfg]
[mcu]
serial: /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_AM00988F-if00-port0

[mcu btt]
serial: /dev/serial/by-id/usb-Klipper_stm32f103xe_36FFD7054158303211750957-if00

[pause_resume]

[display_status]

[virtual_sdcard]
path: ~/gcode_files

[printer]
kinematics: none
max_velocity: 1000
max_accel: 1000

Instead i keep getting errors like this:

mcu ‘btt’: Unable to connect
Once the underlying issue is corrected, use the
“FIRMWARE_RESTART” command to reset the firmware, reload the
config, and restart the host software.
Error configuring printer

I tried to comment each board and run it as single board for both of them and it stars just fine without any problem. But when I keep them both, it wont start.

klippy.log is attached

klippy.log (777.4 KB)

Could you please help me out? I will provide any other info if needed.

I found a problem. The problem was I had another Marlin printer connected too. Once i disconnected the other printer and restarted firmware it works like charm with both MCU. Then i reconnected the marlin printer and it still somehow works. Then i tried to restart linux and it still somehow works all at once.